The continually increasing amount of traffic that communications systems are required to handle has hastened the development of high capacity systems. Even with the increased capacity made available by systems operating between 10.sup.9 Hz and 10.sup.12 Hz, traffic growth is so rapid that saturation of such systems is anticipated in the very near future. High capacity communication systems operating around 10.sup.15 Hz are needed to accommodate future increases in traffic. These systems are referred to as optical communication systems since 10.sup.15 Hz is within the frequency spectrum of light. Conventional electrically conductive waveguides which have been employed at frequencies between 10.sup.9 and 10.sup.12 Hz are not satisfactory for transmitting information at carrier frequencies around 10.sup.15 Hz.
The transmitting media required in the transmission of frequencies around 10.sup.15 Hz are hereinafter referred to as optical signal transmission lines which may consist of a single optical waveguide or a bundle thereof. Optical waveguides normally consist of an optical fiber having a transparent core surrounded by a layer of transparent cladding material having a refractive index which is lower than that of the core. Although the theory of optical waveguides has been known for some time, practical optical waveguides that do not absorb an excessive amount of transmitted light have been developed only recently. U.S. Pat. No. 3,659,915 discloses a low loss optical waveguide comprising a cladding layer of fused silica and a core of fused silica doped with one or more materials that selectively increase the index of refraction of the core above that of the cladding.
To establish between a plurality of stations an optical communication network, i.e., one employing optical signal transmission lines, a variety of interconnection schemes may be utilized. Each station can be "hard wired" to every other station, but when many stations must be interconnected, the excessive amount of optical signal transmission line required causes this method to be undesirable due to both the cost of the transmission line and the space consumed thereby. The stations may be interconnected by a loop or line data bus which drastically reduces the required amount of optical signal transmission line. A loop data bus can be used, for example, to interconnect a plurality of stations, one of which is generally a central processing unit. This type of transmission path has no end, and data, in principle, could circulate around the path many times. In practice, attenuation is large enough that the data is not detectable after one circuit of the loop. Transmission in the loop can be unidirectional, i.e., each station transmits in one direction only, or it may be bidirectional, depending upon the type of coupler used at each station.
Each station requires a coupler for extracting from the transmission line a fraction of the optical signal propagating therein and for injecting onto the transmission line an optical signal of sufficient strength that it is detectable at each of the remaining stations.